The present disclosure generally relates to methods for drilling a borehole in a subterranean formation or like geological structure and, more specifically, to polymer-based drilling fluids and methods for their use in mitigating fluid loss during drilling operations.
Earthen boreholes are drilled for a number of applications including, for example, oil and gas exploration and production, minerals exploration and production, water sourcing, and the like. As used herein, the term “borehole” will refer to any elongated pathway excavated in an earthen structure, such as a subterranean formation, regardless of purpose. Construction-related drilling projects can similarly prepare boreholes suitable for laying pipelines or cables, or establishing locations for footings or pilings of a structure. Although these applications may each involve a drilling process of some form, there may be fundamental differences dictating how the drilling process is conducted in one application versus another.
During drilling operations, a drilling fluid is usually used to cool the drill bit, to control pressure within the borehole, and to suspend and transport drill cuttings from the borehole to the earth's surface. It is usually desirable to control loss of the drilling fluid from the borehole into porous features of the subterranean matrix. Although there are a number of reasons that fluid loss may be undesirable during a drilling operation, weakening of the subterranean matrix by excessive fluid incursion is often an overriding concern.
Fluid loss into the subterranean matrix can usually be lessened by forming a filter cake within the borehole. In most instances, the drilling fluid is formulated with materials that promote formation of a filter cake upon initial spurt loss of the drilling fluid to the subterranean matrix. In oil and gas production, the filter cake can also mitigate the premature incursion of various formation fluids into the borehole during drilling, and the filter cake can thereafter be removed to allow production to commence. In other types of drilling applications, however, it is not necessary to remove the filter cake once drilling of the borehole is complete.
As indicated above and discussed further hereinafter, different types of drilling applications may vary in several aspects. Although drilling fluids may function similarly across various drilling applications, the properties of the drilling fluids are usually adapted to focus on the needs of a particular application. For example, mining applications, particularly mineral exploration applications, tend to make use of smaller boreholes and drill bits than do oil and gas applications. The small annular space present within typical mining applications can make less viscous drilling fluids desirable, such that they can be more easily circulated within the borehole. The highly mineralized formations commonly associated with mining applications can also make tailoring of the drilling fluid desirable to accommodate localized conditions of porosity, pH and temperature that may be present. Similarly, water wells, particularly potable water wells, are frequently limited in the types of materials that can be used in formulating a drilling fluid for the well. In many instances, any materials that are introduced into a water well must be NSF certifiable, and a significant number of chemical agents do not meet this requirement.
Drilling fluids often contain a plurality of bridging particulates that collectively form a fluid-blocking filter cake across pore throats and other porous features on the walls of the borehole. Degradable polymers are commonly used for this purpose, particularly in oil and gas exploration and production, where induced-degradation or self-degradation of the bridging particulates may be desirable upon commencing production operations. Degradable polymers, particularly biodegradable polymers, may also be particularly desirable from an environmental standpoint in such applications.
In other applications, degradable polymers and other degradable materials can be undesirable, and counter intuitively can represent an environmental concern. For example, in water well drilling, polymer degradation products may result in contamination of a water source encroached by the borehole, possibly making the water source unsuitable for use or consumption. Biodegradable polymers lost to the subterranean matrix in such applications can further provide a food source for various bacteria, possibly resulting in formation damage and/or water source fouling from uncontrolled bacteria growth. Related issues can also be encountered in mineral wells and like boreholes. For example, the generation of degradation products in a mineral well can further complicate an already complex mineralogical profile within the borehole.
Some drilling fluids use bentonite or other clay derivatives in order to convey viscosity to the fluid and to promote formation of a filter cake during drilling. The use of bentonite and other clays can prove problematic in a number of instances. In concentrations effective for promoting fluid loss control, clays can often produce drilling fluid viscosities that are too high for effective use in mineral wells and related types of boreholes. Due to their layered molecular structure, clays are also susceptible to swelling in the presence of various ionic materials, such as subterranean brines, which can result in further changes in viscosity. Costs associated with disposal of clay-containing drilling fluids can also be highly problematic, and some locales have even regulated or banned their use.
Polymer-based drilling fluids, particularly those containing substantially non-degradable polymers, may be used as an alternative to clay-based drilling fluids in some instances. Although they may sometimes exhibit comparable rheological performance, polymer-based drilling fluids often display inferior fluid loss control properties compared to clay-based drilling fluids. Without being bound by any theory or mechanism, it is believed that the layered structure of clay particles may result in more effective bridging during filter cake formation than can polymer chains.